A method as well as a plant of the aforementioned kind is well-known i.a. within the cement manufacturing industry. In actual fact, the method described is used in many cement plants around the world where the available cement raw materials mainly consist of soft and wet materials comprising chalk, marl and clay which do not necessarily require grinding or comminution into smaller particles but being capable of being reduced to the desired fineness in a wash mill without any major complications. In kiln systems of the semi-dry type comprising a drier crusher only a portion of the raw materials is prepared into slurry prior to drying while the material components which are already dry or almost dry are fed directly to the drier crusher. Hence the fuel consumption will be significantly reduced in comparison with situations where all raw materials are prepared into slurry.
These soft raw materials often contain a quantity of hard components in the form of flint, sand, marble and the like. These hard components may take the form of relatively large balls of a size equal to a hen's egg and incapable of being crushed to the desired fineness of less than 100 μm in the drier crusher. Typically, the drier crusher will only be able to break down these hard components to a particle size within the range of 1 to 2 mm. The raw materials may also contain strips of hard, compressed limestone which may also be difficult to comminute in the drier crusher. In cases where the raw materials prior to the drying process in the drier crusher are subjected to comminution in a wash mill, the hard material components may relatively easy be screened out subsequent grinding in a separate grinding unit or for subsequent disposal. The drawback of this method is of course the attendant need for all raw materials to be prepared into slurry in substantial amounts of water which must subsequently be removed, for example by filtration or evaporation.